The invention is based on a device for storing heat, comprising a heat storage medium which absorbs heat in order to store heat and releases heat in order to use the stored heat, and a container for holding the heat storage medium, the container being closed by a gastight cover. The invention is furthermore based on a method for storing heat, in which, in a heat exchanger, heat is transferred from a heat carrier to a heat storage medium or, in the heat exchanger, heat is discharged from the heat storage medium to the heat carrier, the heat storage medium being held in a container which is closed by a gastight cover.
Devices and methods for storing heat are used, for example, in solar power stations. By using the device or method for storing heat, solar power stations can be operated uninterrupted even in periods without sun, for example at night. In order to allow uninterrupted operation, large solar power stations require very large heat storage tanks. For example, it is known that in the currently widely operated parabolic trough solar power stations with an electrical power of 50 MW, salt storage tanks are used which contain up to 28,000 t of salt as a heat storage medium. The salt is stored in two dually arranged tanks. Under the effect of sunlight, the heat carrier medium heated in the solar field is driven from the cold tank to the hot tank. When unloading, the heat storage medium is taken from the hot tank and cooled in the power station while generating electrical energy. The cooled heat storage medium is returned to the cold tank.
In order to be able to operate solar power stations with a higher power or over a longer period of time without interruption, much larger heat storage tanks are required compared with the currently known devices for storing heat. In this case, on the one hand, it is possible to use a large number of smaller storage tanks, although this entails a large area requirement, or to use large storage tanks.
In order to prevent a negative pressure from being formed in the container, owing to which unreliably large forces act on the shell of the container, unoccupied volume in the containers is filled with a gas. In the case of oxidizable heat storage media, it is also necessary to avoid oxidation. To this end, for example, nitrogen is used as a gas for occupying the volume not filled with the heat storage medium. In the case of heat storage media which cannot be oxidized, air may also be used for this.
In the event of temperature variations in the storage system, the volume s occupied by the heat storage medium change owing to thermal expansions of the heat storage medium. The volume of the gaseous tank content changes to a particularly pronounced extent in this case. The prevalence of high pressures owing to the design of the container necessitates great outlay. For this reason, additional pressure loads on the shell of the container should be prevented. To this end, large containers are preferably operated at ambient pressure. Currently, the volume change due to the thermal expansion of the heat storage medium is ensured by discharging gas to the surroundings when there is a volume increase. Gas then needs to be resupplied when there is a reduction in the temperature of the heat storage medium. This requires the procurement and provision of gas in order to be able to compensate for corresponding variations. When using a heat storage medium with a high vapor pressure, it is necessary to provide for particularly large gas exchange quantities caused by evaporation in the hot storage phase and by condensation in the cold storage phase.
In the case of oxidation-sensitive heat storage mediums, for example oil, the gas undertakes an inerting function. Even in small concentrations, oxygen can lead to oxidation processes of the heat storage medium, in which case detrimental, for example insoluble products may be formed. Undesired deposits can occur as a result of this. The storage capacity of the heat storage medium is furthermore affected by the formation of insoluble products. Currently, separation of high-boiling components by distillation may be carried out in order to remove such undesired deposits. As an alternative, a heat storage medium in which deposits have formed is replaced. In the case of a combustible heat storage medium, a sufficiently large concentration of oxygen together with a significant vapor pressure can also lead to an explosion risk. In the case of substances not sensitive to oxidation as the heat storage medium, for example when using nitrate melts, the device may for example also be operated with air as a gas to compensate for volume variations.
In particular when a gas overhead with an inert gas is necessary, high operating costs are involved. Liquid nitrogen is currently used for volume compensation in solar power stations, which is evaporated and delivered to the heat storage medium. By using large isobaric gas storage tanks with a variable volume, which are also referred to as gasometers, it is possible to construct a gas buffer system. Owing to the variable volume, the excess gas can be collected during the heating and released during cooling. The use of such gasometers, however, is problematic when using heat storage mediums having a significant vapor pressure, since the heat storage medium condenses into the gasometer. In order to prevent condensation, it is necessary to have coolers which effectively condense out heat storage medium which has evaporated in the gas, and recycle it. These devices, however, are very elaborate to construct and operate.
Furthermore, floating lid tanks are also known. Floating lids, however, generally are not made fully gastight and, for example, have wall seals with the container wall. An improvement of the gastightness is achieved, for example, by the use of a second sealing system. For use with the high temperatures of a heat storage tank, however, this solution is not sufficient. Oxygen can reach the material which is oxidation-sensitive at high storage temperatures and undesired solids, or also undesired inert gases such as carbon monoxide or carbon dioxide in the case of heat storage mediums containing carbon, can be formed by oxidation. When using heat storage mediums containing sulfur, sulfur dioxide and sulfur trioxide can be formed, which can damage the container walls by corrosion.